Instruments suitable for use in a vehicle include clocks, hand held computer games, mobile phones, in fact any instrument which is capable of displaying information electronically. Instruments for displaying information on a vehicle display are usually located on the dashboard of an automotive vehicle, and require some form of illumination to allow the driver to read the information when it is dark. An instrument usually comprises an information bearing surface in a housing having a transparent cover for protection of the instrument. The information bearing surface usually has symbols indicating a scale such as graphics, numbers or a dial, and a pointer for indicating a point within the scale, or may comprise a liquid crystal display suitable for displaying numeric information. The information bearing surface may also include features such as rings defining the perimeter of a particular instrument or gauge. The instrument is generally mounted on a printed circuit board (PCB) which is connected to sensors (directly or indirectly via a network) which monitors certain engine conditions such as speed, temperature and fuel level to name but a few. Conventionally, instruments are mounted together to form an instrument cluster. As customers become more sophisticated in their requirements, it becomes important to provide lighting for vehicle instruments which is both distinctive, and cost effective to produce.
In general, one of two types of illumination is used, either front lighting or back lighting. Front lighting refers to illumination from the viewing side of the information bearing surface which reflects off the information bearing surface. The light need not be perpendicular to the information bearing surface, hence, this term includes side lighting, where the light is reflected from the information bearing surface. The information is printed so that the light reflecting from the symbols is in contrast to the light reflecting from a background, for example the symbols may be black and the background may be white or the symbols may be white and the background may be black. When the symbols are lighter than the background, the illumination is known as negative mode, and when the symbols are darker than the background, the instrument is described as positive mode.
Back lighting refers to illumination of the information bearing surface from the side opposite to the viewing side of the instrument. In this case, the information surface contains opaque or semi-opaque portions so that the light passing through the information bearing surface renders the symbols visible.
Back lighting of vehicle instrument panels has become more common as it is currently considered to provide a better overall appearance for the vehicle instrument panel because it is not required to mount lighting components in front of the instrument. However, front lighting provides a more pleasing appearance of the instrument when illuminated.
Front or back lighting may be achieved either by providing illumination directly from a light source (which may include the use of reflective surfaces), or indirectly from a light source which is coupled to a light guide which directs the light from the light source to the required illumination point.
A problem with known instruments which have an associated light source is that, while front lighting allows a simpler design for the information bearing surface and any associated pointers, any cost savings resulting from these simplified designs are negated by the extra costs required for flexible connectors from the PCB to the light source or complex light guides, and for more complex assembly. Front lighting from a point source also often produces shadows, Back lighting, on the other hand, usually involves the use of a plurality of light sources, and a more complex arrangement is required to illuminate the pointer. Furthermore, applying symbols to the information bearing surface is more complex in the back lighting case because poor distribution of the illumination can cause portions of the instrument to have better illumination than others. This is compensated for by providing a different thickness of opaque or semi-opaque portions in some areas than in others; however, an undesirable effect commonly referred to as “color shift” occurs when using a compensation method, and that is particularly noticeable with white color illumination. The color shift is caused by the difference in the coefficient of transmission of light in distinct areas of the information bearing surface.
A further problem with backlighting is that as light sources, in particular light emitting diodes (LEDs), develop and more light emission becomes possible, distribution of that light in an effective manner becomes more difficult.
It is known to use an optical fiber for illumination of instrument panels, for example DE19940849 discloses an optical fiber for illuminating an instrument panel which has a section for radial emission of light.
It is known to use a light conducting member around the edge of a dial for the purpose of illuminating an instrument. For example, JP2006194613 discloses arc shaped light guide plates; US2006066250 discloses a glass panel and curved tube light guide made of lighting sensitive material. GB482595 discloses a solid rod-like light conducting member around the edge of a dial. The member may be covered by opaque material excepting for two slits on the side facing the dial and may be placed behind the cover.
It is known to provide cuts or discontinuities to allow light to escape from an optical fiber. For example, WO8400800 discloses a flexible transmitting guide which may have cuts at intervals along its surface; JP57063469 describes a device for illuminating a watch which has optical fiber with plurality of cuts for emitting light; discloses an illuminating system having a light guide member with slits.
Finally, it is known to use a cover (or lens) as a light guide. For example, French patent application number FR 2760414 discloses an instrument having an external protection glass which is illuminated by at least one associated light source and acts as a light guide.
US2003112619 discloses an illumination system suitable for illuminating an instrument, comprising an information bearing surface; a substantially transparent protective cover; and a first light source coupled to the protective cover such that the protective cover acts as a light guide; wherein the protective cover is arranged to allow light to escape at a selected location to illuminate a portion of the information bearing surface.
However, when using the entire visible surface of the protective cover as a light guide, the presence of any scratch or contamination such as dust or a finger print on the visible area will allow light to escape and to be visible to the driver.
In order to overcome the problem of light leakage that occurs when light propagates through the visible area of the protective cover, a novel design of protective cover is proposed that propagates light only along its hidden periphery where light cannot be perceived by the user when the visible area is contaminated or contains scratches.
It would be desirable to have an illumination system suitable for illuminating an instrument, wherein the illumination system provides an efficient and substantially uniform illumination of a desired area of the instrument.